Bar codes have been used in a wide variety of applications as a source for information. Typically bar codes are used at a point-of-sale terminal in merchandising for pricing and inventory control. Bar codes are also used in controlling personnel access systems, mailing systems, and in manufacturing for work-in process and inventory control systems, etc. The bar codes themselves represent alphanumeric characters by series of adjacent stripes of various widths, i.e. the universal product code.
A bar code is a set of binary numbers. It consists of black bars and white spaces. A wide black bar space signifies a one and a thin black bar or space signifies a zero. The binary numbers stand for decimal numbers or letters. There are several different kinds of bar codes. In each one, a number, letter or other character is formed by a certain number or bars and spaces.
Bar code reading systems or scanners have been developed to read bar codes. The bar code may be read by having a light beam translated across the bar code and a portion of the light illuminating the bar code is reflected and collected by a scanner. The intensity of the reflected light is proportional to the reflectance of the area illuminated by the light beam. Thus, the scanners read the difference between the light and dark bars by reading the absences of reflected light This light is converted into an electric current signal and then the signal is decoded.
Bar codes have been affixed to many different types of documents, so that they may be read by a machine, thereby reducing labor costs. Documents that include bar codes and/or indicia have been issued by governmental agencies, financial institutions, brokerage houses, etc., that authorize the holder of such documents to perform authorized tasks or grant rights to the holder of such a document. Examples of such documents are drivers licenses, entry access badges, identification cards, etc.
Bar codes and indicia have been printed on documents with transparent or invisible inks in order to place additional information on the documents and/or to make it more difficult to counterfeit the document. A lower layer bar code and/or lower layer indicia may be printed on a document with a fluorescent ink or a conventional ink and a upper layer bar code and/or upper layer indicia may be printed on top of the lower layer bar code and/or lower layer indicia with a invisible fluorescent ink or a conventional ink. The lower layer bar code and/or indicia is read during one period of time and the upper layer bar code and/or indicia is read during another period of time.
One of the problems encountered by the prior art is that the background printing effects the signal to noise ratio of the detection of the invisible printing of the upper layer bar code and/or indicia. The reason for the above is that the detectors that are utilized to detect the emitted signal from the invisible ink of the upper layer bar code and/or upper indicia detect spurious reflections of the illuminated light source. Thus, the detectors have difficulty in determining what light was emitted from the upper layer invisible bar code and/or upper layer invisible indicia and what light was reflected back to the detector from the visible lower layer bar code and/or visible lower layer indicia.
Another problem encountered by the prior art is that paper as well as conventional color inks often contain fluorescent brightners which when illuminated by an excitation source for the purpose of reading the printed material fluoresce and obscure the signal from the invisible ink signal. Thus, it is difficult to read the upper layer invisible bar code and/or upper layer invisible indicia and the visible lower layer bar code and/or visible lower layer indicia, since the signal coming from the upper layer invisible ink is mixed with the background fluorescent signal.